Long-term exposure to hazardous pollutants in the air is a problem in urban areas all over the world. In European countries, where regulations have been sharpened over the years, an estimated 200,000 to 300,000 people die prematurely every year due to bad air quality. Many more people experience negative health effects, like respiratory and cardiovascular diseases. In the Netherlands as well, air quality has improved over the years. Yet still, most of the country exceeds the WHO standards for most pollutants. In an effort to bring the negative impact of air pollution down to 0, many urban planners suggest planting trees as a potential mitigation strategy. However, the impact of trees on air quality is not fully understood.

This thesis investigates how the leaf area density, or LAD, of trees in a street canyon affects the dispersion of pollutants emitted in said canyon. It does so with a Reynolds-Averaged Navier-Stokes model in the OpenFOAM v7 software. A street canyon with an aspect ratio of 1:1 is considered, with a row of trees running through the middle. For four different LAD values, the impacts on canyon concentrations are examined. The results for mean concentrations on the facades and within the canyon are computed and visualised for three different wind directions: one parallel to the canyon, one perpendicular to the canyon, and one at a 45° angle. This was also done for multiple street lengths to investigate the impact of the street boundaries.

The results for this LAD investigation are used to determine monthly averages and seasonal effects. Therefore, the results are subjected to daily KNMI meteorological data. This method utilises wind parameters, direction and speed, to estimate canyon concentrations. Two different types of trees are considered: deciduous trees, which lose their leaves in winter, and coniferous trees, which are evergreens.

It is found that an increase in LAD causes an increase in pollution for parallel wind and angled wind, especially near the boundaries. This is mainly due to the reduced wind speeds within the canyon, limiting dilution. For parallel wind, recirculation zones caused by trees cause the canyon concentrations to accumulate. For perpendicular wind, lower pollutant concentrations are found with higher LAD, due to enhanced vertical transport and reduced accumulation resulting from limited lateral transport.

The seasonal impact of deciduous and coniferous trees is very LAD-dependent when taking yearly averages. Depending on the chosen LAD value, deciduous trees are up to 17-23% worse than a case with no trees, and coniferous trees are up to 28-36% worse, although for low LAD values the impact is small. ... Read more

While reconstructing urban environments, the accuracy of the model is strongly related to the quality of the input data. The initial type of input data for this process could be the footprints of the buildings, which are typically used to reconstruct the buildings, either through manual or automated methods. However, these data could be biased, and when we perform a Computational fluid dynamics (CFD) simulation, they could impact our results. An example of such bias could be either translation or rotation. In our case, simple building footprints were used to define these biases associated with the uncertainty of raw data. Additionally, statistical analysis for representing and quantifying this uncertainty was performed.
The Case C dataset from the Architectural Institute of Japan (AIJ) is an example of a canonical case for our simulations, in which the building footprints could play an important role in the calculation of our results. A CFD simulation on such a canonical case would typically involve the steps of preparing the geometry, generating the mesh, setting boundary and initial conditions, validating the results using experimental data and finally, performing uncertainty analysis. The last step involves various ways of representing the results, like scatter plots, box plots and contour plots. Important aspects of this analysis were the visualization of the flow patterns, the calculation of various quantities of interest such as velocity or turbulent kinetic energy, and the comparison of the simulation results with the experimental data from the AIJ dataset. The effects were examined across multiple wind directions and different footprint uncertainties. This approach could help us to improve the accuracy and reliability of the CFD simulations. ... Read more

Many regions around the world are prone to tsunami risk, and their populations are expected to increase. Moreover, past events like the 2011 Great Eastern Japan Earthquake and Tsunami resulted in numerous fatalities and the failure of many coastal protection structures. These events underscore the urgent need for further research in tsunami engineering and the mitigation of tsunami risk. In fact, the capability of coastal protection structures, such as breakwaters, to withstand tsunami loads is not yet fully understood. Additionally, these structures are typically designed to resist wind waves loads rather than the longer waves produced by tsunamis. This research provides further insight into the interaction between tsunamis and composite breakwaters by analyzing experimental results where a unique technique capable of accurately reproducing tsunamis as scaled N and E-waves was used. The experiments employed a 2-dimensional flume where waves were generated with a tsunami simulator and propagated until they impacted a composite breakwater model, inspired by the world-record breakwater in the Kamaishi bay in Japan. This research thoroughly analyzed the results of one of the experiments conducted at the HR Wallingford research center in the UK, with the objective of understanding the response of a composite breakwater when impacted by a tsunami, focusing particularly on the caisson on top of the structure and its stability. Another objective was the development of a numerical model based on the coupling of the two software OceanWave3D and OpenFOAM, aiming to reproduce physical experiments on tsunami-structure interaction and provide further insights beyond the capabilities of physical tests. The results of this research indicate that the pressures and forces induced by a tsunami on the caisson of a composite breakwater have a dominant hydrostatic contribution. The absence of wave breaking as the tsunami approaches the breakwater and shoals on its rubble mound prevents the generation of impulsive forces on the structure. The analysis also shows that, for the considered tsunami at prototype scale, the caisson would be unstable and fail due to sliding, primarily because of the water level and pressure differences on the two sides of the structure. The model developed in this research demonstrated a good accuracy in representing the physical experiment, as evidenced by elevation and pressure time series, with minor limitations on the lee side of the structure and on its rubble mound. With further validation using additional experimental results, this model can serve as a starting point for future studies on tsunami-structure interaction. It overcomes some limitations of physical testing, potentially provides more accurate results for caisson stability analysis, and offers a cost-effective alternative to physical experiments. ... Read more